This invention relates to apparatus in electrostatographic copiers and printers for fusing tone images to suitable substrates or copy sheets of paper. More particularly, it relates to such a fusing apparatus having a self-learning heater that replaces heat lost by the apparatus to the substrates or copy sheets by varying its temperature control setpoint in direct response to temperature variations due to such heat loss, and thereby substantially preventing "droop" or drops from a desired setpoint in the fusing temperature of the apparatus.
In electrostatographic copiers and printers, it is well known, for example, to use a heat and pressure fusing apparatus for fusing toner images carried on suitable substrates or copy sheets of paper. Examples of such fusing apparatus are disclosed in U.S. Pat. No. 3,945,726, issued Mar. 23, 1976 to Ito et al; U.S. Pat. No. 4,079,227, issued Mar. 14, 1984 to Takiguchi, U.S. Pat. No. 4,487,158, issued Dec. 11, 1984 to Kayson; and U.S. Pat. No. 4,593,992, issued June 10, 1986 to Yoshinaga et al. Typically, such an apparatus includes a pressure roller, a fuser roller that can be made of a metallic core coated with an elastomer, and a heater whose temperature is controlled at a fixed setpoint. The heater is used for warming up and then maintaining the temperature, for example, of the fuser roller at a desired fusing setpoint. However, because the fuser roller ordinarily loses heat to the substrates or copy sheets being run, the actual temperature of the fuser roller, during fusing, usually will vary from such a fusing temperature setpoint.
Conventionally, in an effort to maintain the temperature of the fuser roller at the fusing setpoint, the heater (with its fixed control temperature setpoint) will be turned on or off in response to sensed deviations in the temperature of the fuser roller from such a fusing setpoint. Unfortunately however, because of the thermal inertia, for example, of the metallic cores of the rollers, as well as, the poor conductivity of the elastomeric coating of the fuser roller, the thermal response of the fuser roller to the heater being turned on or off, is usually delayed, and slow. The net result, in the case of a sensed dropping in the temperature of the fuser roller, can be a continued dropping (for a while), even after the heater had already been turned on in response. This characteristically, is what is termed "droop".
In high speed equipment, such continued dropping in the temperature of the fuser roller can result in incomplete fusing, and hence in poor copy quality. In addition to the droop problem, conventional heaters often are not heat efficient since they are themselves controlled at a fixed temperature setpoint, and will (at a given temperature and for a given time interval) ordinarily transfer substantially the same amount of heat to the fuser roller, regardless of any variations in the heat actually being lost by the fuser roller.